1. Field of the Invention
The invention relates to a method and a device for producing classified polysilicon fragments with high purity.
2. Description of the Related Art
Highly pure polycrystalline silicon, referred to below as polysilicon, is used inter alia as a starting material for the production of electronic components and solar cells. It is obtained by thermal decomposition of a gas containing silicon or a gas mixture containing silicon. This process is referred to as chemical vapor deposition (CVD). On a large scale, this process is carried out in so-called Siemens reactors. The polysilicon is thereby created in the form of rods. The polysilicon rods are extracted and comminuted by means of manual methods or using a machine. The coarse polysilicon fragments thus obtained are comminuted further by using crushers. Mechanical crushing methods are described, for example, in EP 1 338 682 A2. The polysilicon fragments are subsequently classified by a screening method, for example a mechanical screening method as known from EP 1 043 249 B1, or optical sorting as is known for example from U.S. Pat. No. 6,265,683 B1. The polysilicon fragments are transported from one device to the next by means of delivery devices, for example conveyor belts, conveyor troughs, vibrating conveyors or transport containers. Each step of processing the polysilicon (for example crushing or sorting) is adapted individually on the basis of the respective specific tasks. In particular, the requirements of the materials used for the device have been adapted individually for each processing or transport step. For the individual devices, the surfaces coming in contact with the polysilicon fragments have preferably been made of low-contamination material for the polysilicon fragments, or of silicon. It is furthermore prior art, when an individual device consists of another material, to clean the polysilicon fragments by a cleaning method (EP 1 338 682 A2 or DE 197 41 465). Depending on the chemicals used, these cleaning methods are to a differing degree capable of removing the metals contaminating the polysilicon from the surface of the polysilicon.